The present invention relates to a lens system including a lens element, the configuration of which is of a parallel plane plate, having a distribution of refractive index which is distributed in a radial direction perpendicular to the optical axis. More particularly, the present invention relates to a lens system in which chromatic aberration is compensated by the lens element.
Conventionally, what is called a radial GRIN lens is variously utilized. In this case, the radial GRIN lens is defined as a lens element having a refractive index distribution in a radial direction perpendicular to the optical axis.
Picture-taking lenses of simple construction for camera use having a refractive index distribution have been conventionally investigated. For example, the above picture-taking lenses are disclosed in the thesis by L. G. Atkinson ("Applied Optics" Vol. 21, No. 18, p 993-998) and the thesis by J. Brain Caldwell ("Applied Optics" Vol. 25, No. 18, p 3351-3355). Further, the technique is disclosed in Japanese Patent Publication Open to Public Inspection Nos. 42109/1992 and 221716/1985.
Examples in which the radial GRIN lens is used as a picture-taking lens of a camera or a video camera, especially examples in which the radial GRIN lens is used as a zoom lens, can be seen in patents which have already been applied. The examples are Japanese Patent Publication Open to Public Inspection Nos. 911/1987, 38711/1989, 15610/1992, 79013/1990, and 85819/1990. In any cases, the lens surface is not formed plane, but it is formed spherical or it is formed to be other curved surfaces.
On the other hand, what is called a radial GRIN lens in which a lens element having a refractive index distribution in a radial direction perpendicular to the optical axis, is applied to an optical laser beam system. The radial GRIN lens can be seen in Japanese Patent Publication Open to Public Inspection Nos. 254923/1986, 91316/1985 and 124011/1988.
The configuration of the surface of a picture-taking lens of the prior art having a refractive index distribution is formed as follows:
Both surfaces of the lens having a refractive index distribution are not plane. At least, one of the lens surfaces is spherical or aspherical. Therefore, with respect to the monochromatic aberration, the degree of freedom of the lens is high compared with a radial GRIN rod lens, both surfaces of which are plane. Accordingly, the performance of the conventional lens can be highly improved. However, from the viewpoint of practical use, the conventional lens is inferior, because it is difficult to machine the lens, and the costs are high.
In the conventional radial GRIN rod lens, both surface of which are formed plane in parallel, as it is applied to an optical laser beam system, consideration is given to the lens only when the refractive index distribution coefficient is a dominant wave length, and consideration is not given to the chromatic aberration of the optical system.
Technique of achromatism is disclosed in Japanese Patent Publication Open to Public Inspection No. 124011/1988. However, in this case, only radial GRIN rod lenses are combined in this lens system, so that the lens thickness is increased 3 to 4 times as much as the lens diameter. For this reason, it is difficult to put the lens into practical use.